When the rotor in a centrifuge, e.g., a laboratory centrifuge or a blood centrifuge, is driven at relatively high speeds, such speeds can, if there is an unbalance, lead to violent vibration of rotor and drive. Therefore, to prevent damage to centrifuges, they are usually equipped with an unbalance detector to shut down the centrifuge on reaching a preselected unbalance threshold.
Unbalance detectors are based in principle on the fact that out-of-balance equipment parts move relative to one another, because the rotating parts are usually mounted on a carrier plate and are spring-mounted relative to the casing or have an elastic drive spindle. Equipment parts then come in contact when the unbalance becomes great enough. If an appropriate sensor is placed at the points of contact, it can detect an unbalance and send a signal via an analysis stage.
The state of the art has a number of unbalance detectors for centrifuges that operate according to this principle and differ mainly in the type of sensor technology.
For example, U.S. Pat. No. 4,491,019 concerns a device for detecting an unbalance in a rotor of a centrifuge, where a stationary magnetic field sensor and a permanent magnet are mounted on the flat part connected to the casing. The rotor has an annular flange made of soft iron opposite the flat stationary part with the permanent magnet, forming a narrow air gap. The magnetic field sensor is arranged so that it responds to changes in field in the air gap due to out-of-balance movements.
This known embodiment is very expensive and can be used economically only for special individual solutions. Because of the narrow air gap, it is also very sensitive to dirt and therefore is not sturdy enough for use under rough conditions. The very narrow air gap also causes a very high display sensitivity, because even very slight unbalances are detected. This extremely high display sensitivity is required with very special applications such as an ultracentrifuge, but it is normally undesirable.
U.S. Pat. No. 4,214,179 discloses a device for detecting an unbalance in a rotor of a centrifuge having as the sensor a rotating, electrically conducting ring concentrically surrounding the drive spindle of the rotor. With a predetermined deflection of the drive spindle due to vibration which occurs with an unbalance, the drive spindle comes in mechanical contact and thus also electrical contact with the ring, thus closing the circuit, and the electric signal derived from this is used to stop the centrifuge drive.
The main disadvantage of this known device is that it needs a sliding contact between the rotating contact ring and a stationary ring for the power supply, which is subject to high wear and is also very susceptible to dirt. Moreover, with this known device, parts of the centrifuge are energized, which is undesirable in certain applications, e.g., in the medical area, because it could endanger the safety of the operating personnel.
U.S. Pat. No. 4,972,110 discloses a device for detecting an unbalance in a rotor of a centrifuge with an adjustable optical sensor consisting of a radiation source and a radiation receiver which are arranged primarily tangentially to the mounting surface of the drive casing. Out-of-balance vibration of the rotor alters the amount of radiation striking the radiation receiver, which can be used to derive a cut-off signal for the drive. This known photoelectric release has the disadvantages of being relatively expensive, very sensitive in its display and susceptible to soiling.
U.S. Pat. No. 3,422,957 discloses a device for detecting an unbalance in the rotor of a washing machine drum or centrifugal drum with a vertical drive axis having as its sensors an arrangement of microswitches which are activated by an excessive deflection of the rotor due to an unbalance in the washing machine drum and then deliver a cut-off signal for the drive motor.
This known device has the particular disadvantage that it is susceptible to faults, not least of all because of the moving switch parts.